Iowa produces 25% of the nation's annual ethanol output of six billion gallons, making it the largest producer in the nation. The use of ethanol as fuel not only reduces the emission of harmful air pollutants, but also helps cut down dependence on imported fossil fuels. Dry-grind corn milling facilities are used mainly where corn is milled, cooked, and fermented in water. However, during fermentation in a corn dry milling facility, only about 30-35% of the corn is actually converted to ethanol, one-third to CO2, and one-third remains as dissolved organics and solids in the whole stillage after distilling the ethanol. The stillage contains about 89% water.
Most of the solids in thin stillage are moved by centrifugation and dried to a product known as distillers dried grains (DDG), which is sold as animal feed. DDG are low in essential amino acids, particularly lysine (about 0.75%), limiting the use to ruminants mainly, while there is a much larger market in Iowa and other agricultural states for hog and chicken feed. The excess water from the centrifuge (centrate), known as thin stillage, contains about 6% organic material and is a major burden for ethanol plants.
An ethanol plant generates about 6 gallons of thin stillage per gallon of ethanol i.e. a typical 50 million gallon per year ethanol plant generates 300 million gallons of thin stillage, i.e. 550-600 gallon thin stillage/minute. Half of the thin stillage is typically recycled to the fermentation process, but the other half of it still has to be treated. Most of the water in thin stillage is evaporated to leave a syrup, which is blended with DDG in a product known as DDG with solubles (DDGS). Evaporation is costly and could add about $0.10 per gallon to the cost of producing ethanol. It generates air pollutants and a condensate rich in organic acids that requires additional treatment for removal before use as process water.
It is therefore a primary objective of the present invention to provide a means of reducing the thin stillage burden to ethanol plants.
It is a further objective of the present invention to provide a means of alleviating the need to treat thin stillage generated during ethanol production by evaporation and subsequent treatment to remove volatile organic compounds.
It is a further objective of the present invention to provide a means of reducing the energy requirements and costs associated with treating thin stillage during ethanol production.
It is a further objective of the present invention to provide a means of treating stillage that does not cause air pollution.
It is still a further objective of the present invention to provide a means of generating revenue from low-value thin stillage, while reducing wastewater purification costs.
The method and means of accomplishing each of the above objectives as well as others will become apparent from the detailed description of the invention which follows hereafter.